Watch hands are manufactured with tight tolerances. The hole in the hand for the passage of the pipe is generally made with a precision of around a micrometre. This precision is necessary in order to ensure that the hand is correctly driven onto its arbour. Hands are generally made from a metal sheet with a thickness of no more than a few hundred micrometres. To reduce the thickness of the watch, the distance between the hands and the watch glass are usually no more than a millimetre, whereas the distance which separates two coaxial hands, for example the hour hand and the minute hand, is on the order of a fraction of a millimetre.
Watch hands evidently have a functional role, for example indicating the current time, but they also have a decorative role by greatly contributing to the aesthetic appearance of the watch to which they are fitted. To this end, the hands must meet strict criteria. A well known technique in the world of watchmaking permitting the hands to be seen in the dark consists in coating the surface of the hands oriented towards the user with a layer of phosphorescent material. One example of such a material is a non-radioactive photoluminescent pigment sold by the Japanese company Nemoto & Co. Ltd, under the registered trademark Super-Luminova®. This pigment may also be used to coat the hour symbols on the watch dial. In daytime, the layer of phosphorescent material absorbs light energy. This light energy is then released at night by the layer of phosphorescent material in the form of luminous radiation. This technique of illuminating the hands of a watch is quite convenient insofar as the layer of phosphorescent material operates in a completely passive manner and therefore does not require actuation of any mechanical or electrical device in order to function, nor does it require any energy to be taken from the watch. However, this technique of illuminating the hands of a watch also has some drawbacks, the first of which requiring mention is the fact that the layer of phosphorescent material must be pre-illuminated by a light source before it is able to transmit light energy. Likewise, the phenomenon of retransmitting light by phosphorescence is limited in time, so that the luminosity of the phosphorescent layer decreases gradually as it releases the light energy that it has stored. The appearance of the hands coated with a layer of phosphorescent material is not, therefore, constant. Finally, there is a limited range of phosphorescent materials available on the market, so that most watches in which such materials are used generally have the same appearance in the dark.
As a variant of the phosphorescent layer, it is possible to envisage using active illumination techniques in which discrete light sources are used to light the hands of a watch. By way of example, it is possible to integrate an ultraviolet light source in the watch, so that the radiation emitted excites the fluorescent material which coats the hands. However, this type of embodiment also has drawbacks. On the one hand, ultraviolet light sources, also known as black light sources, emit a certain amount of visible light, so that the user is likely to see a halo of light at the place where the light source is placed. On the other hand, given that the hands can move in relation to the ultraviolet light source, they do not always receive the same quantity of ultraviolet light, so that their luminosity may vary. A third drawback lies in the fact that the conversion efficiency between visible light and ultraviolet radiation is mediocre.